Accessing chiral NHC-boranes by abstraction of C₆F₅ from the B(C₆F₅)₄⁻ weakly coordinating anion

Abstract

The prolonged heating of an N-heterocyclic carbene-stabilised borenium cation resulted in the abstraction of a –C₆F₅ group from its B(C₆F₅)₄⁻ counter-anion. The resulting new chiral NHC-boranes presented an unusual cis/trans isomerism around the newly formed plane. The corresponding borenium cation, due to its Lewis superacidity, is a potentially interesting new chiral organocatalyst.

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Boron cations have taken an important place in catalysis since the seminal work of Corey on cationic chiral oxazaborolidinium catalysts (Fig. 1, 1a) and have gained increasing interest during the past decades with the development of Frustrated Lewis Pair (FLP) catalysed hydrogenations and of electrophilic borylations.^1,2 N-Heterocyclic carbene (NHC) stabilised borenium cations are appealing for their ability to heterolytically cleave dihydrogen and promote imine reduction in FLP systems.³ They have also been shown to enable the reduction of unsaturated compounds and to borylate methane thanks to their strong Lewis acidity.⁴ During the past decades, several examples of chiral NHC-boranes were reported, either through the sole coordination of four different ligands at the boron centre (1b, 1c),⁵ by the introduction of C-centered chiral centres (1e)^6a or by both methods leading to the existence of diastereoisomers (1d, 1f).^5h,6b The corresponding C-centered chiral borenium cations of 1e and 1f were used in enantioselective reduction of imines with moderate enantiomeric excesses.⁶

Fig. 1 Examples of a described chiral borenium cation and of chiral NHC-boranes, NTf₂⁻ = N(SO₂CF₃)₂⁻.

With the intent of expanding the family of pyramidalised boron species, borenium cation 5 was targeted to increase the Lewis acidity from both the pyramidalisation and the cationisation of the boron centre.⁷ NHC-borane 2, bearing a benzhydryl group, was designed and synthesised to undergo two consecutive C–H borylation reactions leading to the desired cation 5 (Scheme 1).⁸ Reacting NHC-borane 2 with the tritylium salt [CPh₃][BArF₂₀] (BArF₂₀ = B(C₆F₅)₄⁻) led, as expected, to borenium cation 3 through a first intramolecular C–H borylation reaction of a phenyl group of the benzhydryl moiety (Scheme 1). This formation was witnessed by the spontaneous evolution of a gas (presumably H₂) and by ¹¹B NMR as the mixture gave a broad singlet at 54.4 ppm (see the SI, Fig. S-19). Upon heating, a second C–H borylation reaction was attempted to form the pyramidal borenium cation 5. However, while 5 was not isolated nor observed, ¹¹B NMR of the reaction mixture revealed a doublet at −25.9 ppm.

Scheme 1 Synthesis of 4-trans and 4-cis from BArF₂₀ abstraction, and structure of targeted pyramidal borenium cation 5.

After treatment and purification, 4-trans and 4-cis were isolated in 22% and 11% yields, respectively. These compounds result from the abstraction of a –C₆F₅ group from the BArF₂₀ counter-anion of 3, releasing B(C₆F₅)₃. This kind of abstraction of a fluorophenyl ring has occasionally been reported on a B(3,5-(CF₃)₂C₆H₃)₄⁻ (BArF₂₄) counter-anion using some transition metals and a cationic aluminium compound but has, to our knowledge, never been reported at a boron centre.⁹ Interestingly, the C–H borylation followed by the abstraction resulted in the creation of a C-centred and a B-centred stereogenic centres. More than belonging to the family of NHC-boranes coordinated by four different ligands, 4-trans and 4-cis present a clear cis/trans isomerism around the newly formed 6-membered ring. This isomerism around a boron centre has been reported by Curran (1d) and Chuzel (1f) but, to our knowledge, no other examples have been described.^5h,6b

Crystallographic data revealed similar C22–B and C13–B distances for 4-trans and 4-cis (1.598(2) vs. 1.600(2) Å and 1.613(2) vs. 1.612(2) Å respectively) (Fig. 2 and Table 1). However, the C14–B distance was elongated in the case of 4-trans (1.647(2) vs. 1.638(2) Å). For 4-trans, the boron atom is 0.237 Å out of the (C10C11C21) plane, while it is only 0.121 Å away for 4-cis. This difference is due to steric repulsion between the –C₆H₅ and –C₆F₅ groups in 4-cis forcing the boron atom into the plane as C5 and F5 are only 3.674 Å apart. This repulsion forces C7 out of the plane for 4-cis compared to 4-trans (0.131 vs. 0.047 Å respectively). The steric repulsion is also accountable for the greater C22^B^C14 angle observed for 4-cis (112.2° vs. 107.9°). The doublet at −25.9 ppm observed in ¹¹B NMR for both compounds and the quartets observed at 3.55 ppm, for 4-trans, and 3.51 ppm, for 4-cis, in ¹H NMR are consistent with reported NHC-boranes.^4a,5

Fig. 2 X-ray diffraction structures of 4-trans (P2₁/c space group, R-factor: 2.91%) and 4-cis (P2₁/c space group, R-factor: 3.49%), ellipsoids at 50% probability; only one enantiomer is represented.

Table 1 Atomic distances and angles in 4-trans and 4-cis from X-ray diffraction structures

	C22–B (Å)	C13–B (Å)	C14–B (Å)	C5⋯F5 (Å)	Plane⋯B^a (Å)	Plane⋯C7^a (Å)	Angle C22^B^C14
a Plane (C10C11C21).
4-trans	1.598(2)	1.613(2)	1.647(2)	—	0.237	0.047	107.9°
4-cis	1.600(2)	1.612(2)	1.638(2)	3.674	0.121	0.131	112.2°

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DFT calculations of the diverse involved mechanisms were performed. 3 was indeed formed from a C–H borylation reaction of the corresponding borenium cation of 2 (see the SI, Fig. S-1). A 4-centered transition state was found, in accordance with the literature.¹⁰ Calculations showed that the second C–H borylation, at 3, forming the pyramidal borenium cation 5 presented an extreme activation barrier of 208 kJ mol⁻¹ (Fig. 3). Concerning the formation of 4-cis and 4-trans, two ion pair complexes (IPC), with the BArF₂₀ anion, presenting the lowest energies were obtained (Fig. 3). There, the BArF₂₀ anion was either on the same side of the phenyl group (IPC-cis, Fig. 3) or on the opposite side (IPC-trans). IPC-trans and IPC-cis were found to be in the same energy range with a small difference of only 4 kJ mol⁻¹.

Fig. 3 Selected part of the DFT-estimated Gibbs free energy profile (kJ mol⁻¹) for the reaction mechanism of formation of 5, 4-trans and 4-cis in mesitylene (M06-2X/6-311G(d), IEF-PCM (solvent = mesitylene)).

The abstraction of the –C₆F₅ group from the BArF₂₀ anion proceeds in a concerted way, similarly to the –C₆H₅ group abstraction of a BPh₄⁻ anion by a phosphenium cation described by A. Růžička et al.¹¹ The ipso carbon of the abstracted –C₆F₅ group attacks the cationic boron centre of 3, leading to the abstraction. The formation of 4-cis presents an activation barrier of 139 kJ mol⁻¹, higher than the one required for the formation of 4-trans, 131 kJ mol⁻¹. 4-trans was found to be slightly lower in energy than 4-cis, by only 3 kJ mol⁻¹. The formations of 4-trans and 4-cis are both exergonic with global ΔG⁰ of −15 and −16 kJ mol⁻¹ respectively. These calculations indicate that the formation of 4-trans vs. 4-cis is driven by kinetic control of the reaction as 4-trans was obtained in a higher yield than 4-cis.

Intending to obtain these new chiral NHC-boranes effectively, the C–H borylation was performed on 6, an NHC-chelated Lancaster borane, synthesised from the corresponding imidazolium salt. The reaction of 6 with the tritylium salt [CPh₃][BArF₂₀] as a hydride abstractor led to the release of a gas (presumably H₂). After the gas release stopped, the reaction was quenched with a hydride donor, Me₃N·BH₃, which led, after treatment and purification, to 4-trans and 4-cis in 50% and 11% yields, respectively (Scheme 2). These higher yields are explained by the absence of –C₆F₅ abstraction in the reaction and the direct obtention of the compounds after C–H borylation and reduction. Interestingly, the hydride addition proceeded with high diastereoselectivity, in a 5 : 1 ratio, in favour of the trans isomer. The addition of a hydride to the same side as a bulky group on a borenium cation was witnessed by O. Chuzel et al.^6b The corresponding NHC-borenium cation, 7, was subsequently formed, from 4-trans and 4-cis, by hydride abstraction using, once again, the tritylium salt. The newly formed borenium cation presented a ¹¹B NMR signal at 51.6 ppm in CD₂Cl₂ but was not isolated. Its Lewis acidity was measured using the Gutmann–Beckett method and led to the obtention of two close signals at 84.2 and 84.0 ppm in ³¹P NMR corresponding to the formation of the two possible diastereoisomers 8-cis and 8-trans (Scheme 3).^12,13 The shifts of 33.0 and 32.8 ppm of the phosphorus nucleus compared to the free OPEt₃ are in the range of Lewis acidities obtained for NHC-borenium cations.^4a,8b,13 The 84.2 and 84.0 ppm signals integrated in a 1 : 5.5 ratio presumably in favour of the less hindered 8-cis adduct. The fluoride ion affinity (FIA) values were computed with the fluoride groups in both the trans and cis positions compared to the phenyl group.¹⁴ Similar FIA values of 741 and 744 kJ mol⁻¹ were obtained for the 9-cis and 9-trans isomers, respectively (Scheme 3). As the FIA values are above that of SbF₅ (500 kJ mol⁻¹), 4-cis and 4-trans are classified as Lewis superacids.¹⁵ These values are again comparable to those of known NHC-borenium cations.¹⁶

Scheme 2 Synthesis of 4-trans and 4-cis from C–H borylation of 6.

Scheme 3 A) Formation of borenium cation 7, structures of 8-cis and 8-trans, OPEt₃ adducts for the Gutmann–Beckett experiment and obtained ³¹P NMR chemical shifts (CD₂Cl₂, 300 K). δ³¹P(free OPEt₃) = 51.2 ppm (CD₂Cl₂, 300 K). B) DFT calculated adducts for FIA determination. Calculated FIA values for NHC-borenium cation 7 (DFT at the M06-2X/6-311G(d) level of theory).

The racemic mixture of NHC-borenium 7 was combined with enantiopure Kwon's chiral phosphine [P] so as to perform a chiral derivatisation experiment (Scheme 4). The aim was to demonstrate the retention of the C-chiral centre, while the B-centred one was lost during the formation of the borenium cation, by the formation of the four different diastereoisomers of 10 (Scheme 4). After addition of Kwon's phosphine, only three signals were observed in ³¹P NMR, two major ones at 19.6 and 17.9 ppm and a minor one at 15.0 ppm. Nonetheless, ¹⁹F NMR revealed the presence of four different –C₆F₅ groups linked to the boron centres (see Fig. S38). The signals corresponding to the ortho-fluorines of 10 were obtained in a 1 : 1.5 : 1.5 : 1 ratio, proving the formation of the four possible diastereoisomers 10-R-cis-[P], 10-R-trans-[P], 10-S-cis-[P] and 10-S-trans-[P].

Scheme 4 Chiral derivatization of a racemic mixture of borenium 7 using enantiopure Kwon's chiral phosphine [P].

While expecting a new pyramidal Lewis acid, the prolonged heating of a borenium cation resulted in the unexpected abstraction of a –C₆F₅ group of its BArF₂₀ counter-anion. This abstraction was never reported at a boron centre and led to the obtention of new chiral NHC-boranes. These new compounds, more than belonging to the family of chiral NHC-boranes, present a structurally interesting cis/trans diastereoisomerism around the newly formed plane. The corresponding NHC-borenium cation was formed and its chirality was confirmed through a chiral derivatisation experiment. This new chiral NHC-borenium cation was shown to be Lewis superacidic and could be an interesting candidate for enantioselective borenium-catalysed reactions as the chiral centre lies close to the boron centre. Moreover, the C–H borylation method should easily be tuneable for the obtention of different groups on the chiral carbon centre. The use of this chiral NHC-borenium cation and the introduction of different groups through a C–H borylation reaction are currently being studied in our laboratory.

Author contributions

N. N. performed the different syntheses and proofread the manuscript. A. I. A. performed the DFT determination of the mechanism. N. T. performed the XRD analysis. B. C. acquired funding, supervised A. I. A. and proofread the manuscript. G. B. acquired funding, helped in the writing of the manuscript and proofread the manuscript. K. B. designed the study, performed the preliminary experiments and FIA calculations, supervised N. N. and wrote the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). See DOI: https://doi.org/10.1039/d5dt02919h.

CCDC 2495929 (2), 2495930 (4-trans), 2495931 (4-cis) and 2495932 (6) contain the supplementary crystallographic data for this paper.^17a–d

Acknowledgements

The authors acknowledge the European Research Council (ERC, B-yond, grant agreement: 101044649, G. B.) and the Fond National de la Recherche Scientifique (F.R.S.-FNRS) for financial support (grant numbers: T.0012.21 (G. B.) and GEQ UG02222F (J. W.)). The calculations were performed on the computers of the “Consortium des Équipements de Calcul Intensif (CÉCI)” (https://www.ceci-hpc.be), including those of the “UNamur Technological Platform of High Performance Computing (PTCI)” (https://www.ptci.unamur.be), for which we gratefully acknowledge the financial support from the FNRS-FRFC, the Walloon Region, and the University of Namur (Convention No. U.G006.15, U.G018.19, U.G011.22, RW1610468, RW/GEQ2016, RW1117545, RW2110213, and RW2210148, B. C.). The authors thank the PC2 (UNamur) technological platforms for access to all characterization instruments.

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Footnote

† Both authors contributed equally.

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